I would plan on the water temps to be between 14-17C. Different people feel cold in very different ways. I just fished a two days pairs event over this weekend in 15C water & after 5 hours of swimming I was still warm with a 5mm open cell top & 3mm bottoms, my partner in the pairs was cold after a couple of hours & this certainly effected his ability. He could have used a 7mm top & I may have had more utility from a good 3mm open cell top, but it was nice to be warm.
If I was going down to Tassie over that period I would just use a 5mm top & 3mm bottoms as this would give me a few hours comfort even at 14C, if you are inclined to feel the cold a 7mm top might be better. I dislike having bottoms heavier than 3mm for spearfishing as it slows me down too much & for this reason would choose a 7mm top & 3mm pants over a 5mm top & bottom for a bit more warmth. If you spear one of those "tassie trumpeters" please be sure to post some photos.